This invention relates generally to a protective system adapted to isolate a d-c power supply from an external load when the current drawn from the supply is excessive, and more particularly to a system which upon sensing an overload condition then acts during repetitive sampling cycles to intermittently cut off and reconnect the delivery of current to determine whether or not the overload condition is transitory in nature, the system permanently cutting off current delivery should the overload condition exist at the conclusion of the sampling phase interval.
Many forms of military aircraft are provided with guided missile launchers which are mounted on the wing tips or attached to the fuselage. The launcher is permanently secured to its aircraft mounting and is coupled by an umbilical cable to the missile. A connection between the cable and the missile is effected by a releasable coupler such that when the missile is fired and the missile slides off the launcher, the coupler is automatically disengaged to free the missile.
In one known form of launcher, the launcher includes two regulated direct-current power supplies, both of which are operated by an alternating-current generator in the aircraft. One power supply delivers 25 volts with a 3-amp maximum current through the cable to circuits contained in the missile, while the other delivers 175 volts with a 0.2-amp maximum current.
In some instances, because of a defective cable coupler, a decoupling action fails to take place when the missile is fired, as a consequence of which the cable which remains linked to the launcher is ruptured by the departing missile. Such cable rupture may result in a short circuit across one or both of the power supplies. The power supplies in the launcher are potted and shorts thereacross result in overheating of the supplies and the destruction thereof. A short may also occur prior to launching because of a defect in the missile circuits coupled by the cable to the supplies, with the same damaging consequences.
To avoid such damaging effects, there is disclosed in my prior U.S. Pat. No. 4,096,540 a protective system for a regulated d-c current power supply which acts to disconnect the power supply from the load associated therewith in the event of a short circuit or an excessive load having an equivalent effect without, however, in any way impairing the normal operating characteristics of the supply. In this prior art system, a protective relay circuit acts to disconnect the supply from the load when an excessive current is sensed and to automatically reset the connection when the overload condition is cleared up. Where two separate power supplies are involved, the protective circuits are interlocked such that if either relay is actuated, both supplies are disconnected from their missile load circuits.
In my copending application Ser. No. 858,371, filed Dec. 7, 1977, there is disclosed a solid-state protective circuit for isolating a power supply from an external load when an excessive load current condition is sensed. Since semiconductors are used in this protective circuit, the action thereof in response to an overload condition is extremely rapid. Hence a condition lasting for less than one millisecond is sufficient to activate the protector.
With certain types of missiles, a protective system of the type disclosed in my prior patent or in the above-identified copending patent application cannot be used. The inability of these prior systems to protect particular types of missile power supplies is not due to any defect or malfunction thereof but to their inherent lack of ability to cope with factors peculiar to these missiles.
Though one of the two missile power supplies provides a low voltage of +25.2 V and is intended to operate normally with a maximum current drain of 3 amps, when the supply current is first delivered, the start-up drain for a fraction of a second is about 4.5 amperes. Since the prior art protective circuits are designed to cut-off at a given threshold which in this instance would be 3 amperes, the start-up drain of 4.5 amperes would be sensed as an overcurrent condition. Hence the prior protective systems would act to isolate the supply from the load even though the start-up condition does not represent an abnormality but a characteristic of the missile arrangement.
As a consequence, prior protective systems, when used in conjunction with missiles whose start-up drain substantially exceeds the normal maximum current drain, act to prevent the missile from being powered up; for each time primary power is applied to the power supply, the protective system is activated to isolate the d-c power supply from the load. Once activated, primary power must be removed from the power supply in order to reset. If, therefore, a protective system of the prior type were activated in flight, it may not be possible to remove primary power, and the missile would not be able to complete the aircraft mission.